A number of flow measurement techniques are currently used to calibrate mass flow controllers. Primary flow measurement techniques, which derive their accuracy from constants of nature or from other primary measurements such as mass and time, include gravimetric techniques, constant pressure techniques and constant volume techniques. The first primary technique, gravimetric measurement, involves measuring the amount of mass either gained or lost over a time interval. Gravimetric techniques are generally sufficient for measuring higher rate flows where mass loss is significant, but suffer shortcomings in measuring lower rate flows because they typically have insufficient resolution to measure mass flow rates on the order of micro-moles per minute.
The next primary technique, the constant pressure technique, uses a variable volume chamber to keep gas pressure constant. The mass flow is measured based on gas state equations with the mass flow rate depending on the change in volume over time. This technique can work well over a range of mass flow rates, but may require an elaborate system for controlling the volume of the pressure chamber. Thus, the constant pressure technique suffers limitations in calibrating mass flow controllers because constructing a constant pressure chamber (i.e. a variable volume chamber) can require a significant number of moving parts that can cause mechanical complications. As the range of mass flow rates for which the constant pressure chamber is configured increases, the complexity and cost of the constant pressure system will also increase.
The third primary flow measurement technique, the constant volume technique, relies on similar state equations as the constant pressure technique, but the mass flow rate is dependent on the change in pressure, rather than volume, over time. This technique has become popular for calibrating mass flow controllers because of the simplicity of the system (i.e. there are few moving parts). Again, however, the constant volume technique suffers deficiencies for calibrating mass flow controllers because the constant volume technique can typically only be used over a small range of mass flow rates. This limitation exists because, if the flow rate is too high for a given chamber volume, pressure changes associated with the high mass flow rate will be too abrupt to be accurately measured and may quickly exceed the safety limitations of the chamber. Although larger constant volume chambers can be constructed, practical considerations of safety, space and cost establish an upper limit on the capability of this flow measurement technique.
In general, the disadvantage of primary flow measurement techniques is that each technique is limited to a particular flow range where that technique's uncertainties and design limitations are best suited. For this reason, users must typically employ multiple independent flow calibrations systems utilizing several different primary techniques to cover the range of flows for industrial measurements. Alternatively, users can employ independent secondary techniques, such as sonic nozzles (also known as critical flow venturis or critical flow nozzles), laminar flow meters, ultrasonic flow meters, coriolis flow meters, thermal mass flow meters and others. These can be used over a range of flow rates but must be continually calibrated.